ICHTHYOLITH NEODYMIUM ISOTOPE COMPOSITIONS SUGGEST SOUTHERLY INCURSION OF BOREAL WATER MASS ALONG EASTERN MARGIN OF WESTERN INTERIOR SEAWAY DURING CAMPANIAN COOLDOWN

The Campanian stage (83.6-72.1 Ma) is characterized by a cooling of Late Cretaceous climate and a global reorganization of ocean circulation. This episode of ocean-climate change is recorded by marine strata of the Niobrara Formation and overlying Pierre Shale exposed along the Manitoba Escarpment in southwestern Manitoba and northeastern North Dakota. These formations were deposited as part of the Niobrara, Claggett, and Bearpaw transgressions on the passive eastern margin of the Western Interior Seaway (WIS). Despite being deposited as transgressive sequences in similar neritic facies (inner to outer shelf), the lithological character and fossil faunas of the two stratigraphic units differ markedly. Microfossil assemblages within the chalky strata of the upper Niobrara Formation (early Campanian) are dominated by planktic foraminifers, whereas those associated with the siliciclastic strata of the Pierre Shale (mid-Campanian) are composed almost exclusively of radiolarians. It has been posited that this shift in microfossil assemblages signals a major change in ocean circulation where the calcareous assemblages of the Niobrara Formation reflect relatively warm Tethyan waters, while the siliceous assemblages of the Pierre Shale (Millwood Member) are diagnostic of cooler, boreal waters. Here we use neodymium (Nd) isotope compositions of fossil fish teeth and bone (ichthyoliths) as a water mass tracer to test for an inferred change in ocean circulation. Niobrara ichthyoliths (n = 8 samples) yield a mean ε_Nd (t) value of -7.7 ± 1.3 (2 SD) consistent with a Tethyan affinity, whereas those from the Millwood Member of the Pierre Shale (n = 2 samples) yield a more radiogenic mean ε_Nd (t) value of -4.9 ± 0.8 (2 SD) similar to that of the Pacific Ocean. Together, the increases in ε_Nd (t) values and radiolarian abundances across the Millwood Member indicate that cooler boreal waters from the circum-Arctic region displaced warm Tethyan waters along the eastern margin of the WIS during the early stages of the Bearpaw Transgression. This finding suggests that sharp increases in radiolarian abundances at other localities within the Western Interior Basin are useful for tracking the southerly migration of an oceanic front separating warm Tethyan and cool boreal waters as global climate cooled during the Campanian.